Engine ignition systems may include a spark plug for delivering an electric current to a combustion chamber of a spark-ignited engine to ignite an air-fuel mixture and initiate combustion. Based on engine operating conditions, spark plug fouling can occur wherein a firing tip of the spark plug insulator becomes coated with a foreign substance, such as fuel, oil, or soot. Once fouled, the spark plug may be unable to provide adequate voltage to trigger cylinder combustion until the spark plug is sufficiently cleaned or changed.
In areas with poor fuel quality control, spark plug fouling caused by hot spark plugs is a significant issue. Fuel additives such as methylcyclopentadienyl manganese tricarbonyl (MMT) are added to fuel in some countries to raise the octane level. However, the manganese in the fuel additive can coat the spark plug by building up electrically conductive and thermally insulating deposits on the spark plug ceramic. Such build up may cause pre-ignition (PI), and consequently engine damage. The build-up can also cause misfires. The spark plugs can also hot foul leading to rim firing. The rim firing can in turn lead to late burns, where fuel is not fully combusted in the combustion chamber, and burns into the exhaust manifold. The late burn contributes to the presence of increased residuals in adjacent cylinders, further increasing the likelihood of misfire and pre-ignition events. The manganese from the MMT can also coat exhaust oxygen sensors and contaminate precious metal bricks inside an exhaust catalytic converter. This results in eventual degradation of exhaust catalyst efficiency and the need for frequent catalyst replacement. Further, the accumulation of fuel additive may not be easily removed.
In one example, the above issues can be addressed by a method for inferring spark plug fouling due to accumulation of fuel additives thereon. The early detection enables appropriate mitigating steps to be taken in a timely manner, thereby pre-empting engine degradation. One example method comprises inferring spark plug fouling due to accumulation of fuel additive based on actual exhaust temperature being higher than estimated exhaust temperature, the estimated exhaust temperature based on engine operating conditions including air-fuel ratio, spark timing, and EGR. In this way, elevated exhaust temperatures can be better correlated with late combustion via MMT fouling of spark plugs.
As an example, an exhaust temperature may be modeled based on engine operating conditions. Therein, a flange temperature may be estimated based on engine speed and load, and further modified as a function of spark retard from MBT, air-fuel ratio, and EGR schedule. The estimated temperature may compensate for late combustion timing due to spark retard. An actual exhaust temperature may also be measured by an exhaust temperature sensor. If combustion occurs later than predicted due to fuel additive accumulation (e.g., during rim firing), the actual exhaust temperature may be significantly higher than the expected temperature (e.g., greater by more than a threshold amount). Thus, in response to the actual exhaust temperature being higher than the expected temperature, potential spark plug hot fouling due to fuel additive accumulation may be indicated. If there is a concurrent increase in an engine misfire rate or an engine pre-ignition rate, spark plug hot fouling may be confirmed with a higher confidence factor.
In this way, spark plug fouling due to fuel additives may be reliably identified. The technical effect of inferring spark plug hot fouling based on higher than expected exhaust temperatures is that excess exhaust heat from late combustion phasing may be more accurately attributed to the effect of MMT on a spark plug. By correlating the unexpected rise in exhaust temperature with a concurrent increase in the incidence of abnormal cylinder combustion events (such as pre-ignition, misfire, or knock) spark plug fouling due to fuel additive accumulation may be more reliably distinguished from spark plug fouling due to soot accumulation, or due to late combustion timing. In addition, exhaust over-temperature conditions can be monitored and addressed in a timely manner. The approach allows spark plug hot fouling to be accurately deduced without needing to rely only on complex and costly approaches (e.g., switching current measurements). By providing spark plug replacement recommendations based on evidence of malfunction or degradation, rather than a predetermined period of time or amount of vehicle usage, spark plug change recommendations may not be provided too soon, lowering overall vehicle operational costs for the driver, while also reducing the risk of engine pre-ignition. By diagnosing spark plug health, engine life is extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.